This invention relates to a private branch exchange (PBX) used for information exchange between terminals such as telephone sets and, more particularly, to a data transmission system which can be suitably used for such a PBX.
As an example of the PBX, a distributed control type private branch exchange is well known in the art. In this exchange, a plurality of line/trunk modules (or shelves) and a common control module are packed into a case. Each line/trunk module has a plurality of line/trunk cards, and a plurality of telephone sets or data terminals are coupled to each line/trunk card. The common control module has local central processor unit (LCPU) cards corresponding to the respective line/trunk modules, a main central processor unit (MCPU) card and a time switch (TSW) card. The line/trunk cards of each line/trunk module are coupled to the corresponding LCPU card in the common control module through a control data highway and also coupled to the TSW card through a PCM highway. The TSW card is used for switching voice data from line/trunk cards through PCM highways. Each LCPU effects communication of control data with the line/trunk cards of the corresponding line/trunk module through the control highway. The line/trunk card is provided for each of different kinds of terminals such as standard telephone sets and digital telephone sets.
In general, with this system, a LCPU card is arranged to transmit control data to each of corresponding N line/trunk cards through the control data highway in synchronism with a transition between time slots within one frame period of time. Each of the N line/trunk cards, on the other hand, is arranged to transmit control data to the LCPU card through the control data highway only in a time slot allocated thereto in one frame period of time.
The transmission of data from the N line/trunk cards to the LCPU through a common transmission line requires prevention and detection of collision of data on the transmission line. When a line/trunk card transmits a signal in the allocated time slot as described above, it is possible to prevent collision of signals using a simple construction. In this case, however, the efficiency of the transmission line is reduced.
In a case where collision of signals is permitted, the efficiency of the transmission line is increased. In this case, however, a complicated processing such as the detection of collision and retransmission control of signals is required. This is disadvantageous from the standpoints of the reliability and cost of the system.
Heretofore, telephone sets having a simple function have been utilized as terminals of an exchange system. Therefore, greater attention has been paid to the reliability and cost rather than to the transmission efficiency. It is forecast that mulit-function telephone sets and data terminals will be increasingly utilized as terminals of the exchange system. The utility of such terminals necessitates an increased transmission capacity. However, merely increasing a transmission speed for the increased transmission capacity will increase not only the difficulty in the system design but also the cost and power dissipation. The utilization of a collision detection system would cause, on the other hand, a problem in the reliability and the cost of the system as mentioned above.